The present invention relates to an improved air lifting and balancing unit and more particularly to a brake structure and pneumatic control circuit therefor.
By way of background, ball screw type of air lifting and balancing units are known. Briefly, in units of this type pressurized air is supplied to a cylinder to move a piston which acts through a ball screw which, in turn, rotates a ball nut having a drum thereon which in turn lifts a chain or a cable to which a load is attached. If there should be a loss of load from the end of the chain or cable, the latter will whip in an unpredictable manner to possibly cause injury to a workman or equipment. Insofar as known, in the past there was no braking structure associated with an air lifting and balancing unit for braking the drum to prevent the whipping. Additionally, insofar as known, in the past when load lifting was effected by supplying pressurized air at a substantially constant pressure but at a variable volume, the load could be lifted at different speeds by the operator. Thus, the load could be lifted too rapidly or too abruptly, which in the latter two instances could create abrupt shocks to the load or undue stresses to the air balance and to the chain. Also, the speed of lifting fluctuated greatly when there were changes in the supply pressure, which, in turn, often resulted in undesired accelerations of the chain during lifting. To overcome those problems, adjustable needle valves were used to limit the lifting speed, but this caused heavier loads to be lifted too slowly. It is with overcoming the foregoing deficiencies of prior art air balancing and lifting units that the present invention is concerned.